Factory unlocked means the iPhone is legitimately unlocked in the Apple data base. Non-factory unlocked means iPhone is unlocked using hacking technique such as jailbreak or by use of simcard. Non-factory unlocks are less stable, may void warranty and will be lost or not work after upgrading the ios firmware.

A locked iPhone means you can only use SIM card from the carrier the phone is locked to. For example a locked iPhone purchased from AT&T would not work with SIM card from any other phone carriers. An unlocked iPhone has no such restriction.

Jailbreaking allows users to install any software on iPhone, unlocking allows users to use SIM card from any carrier on iPhone. For iPhones that are not factory unlocked, jailbreaking the iPhone is necessary before the unlocking software can be installed to unlock the iPhone.

"FaceTime" is Apple's version of video call. Currently FaceTime is only available in areas with Wi-Fi internet connection and only between iOS devices (iPhone, iPod, iPad and Macs) equipped with front facing cameras.

While most cases do fit, not all will work perfectly, largely depending on how tightly fit the cases are to the phone. iPhone 4S retain almost exactly the same dimension as the previous iPhone 4. However, there are slight change in locations of some components, most notably the location of the buttons on the side and the proximity sensor, all of which differ from previous model by a few millimetres.